U.S. government and Boeing Co. experts gave good grades to the design and certification of the company's 787 Dreamliner, though they recommended improvements to supplier oversight.

Prepared in the wake of two lithium-battery fires that sparked international accident investigations and grounded the Chicago plane maker's flagship model for several months last year, the long-awaited report determined that the Dreamliner "met its intended safety level." The global fleet's reliability during the first 16 months of...